Hormone-dependent breast carcinomas regress when deprived of endogenous estrogens. In postmenopausal women with breast cancer, removal of the adrenals causes tumor regression even though plasma estradiol levels are very low (15 pg/ml) prior to surgical ablation. The concentrations of estrogen in the tumor, however, are orders of magnitude higher (i.e. 500-1000 pg/gm wet weight) than in plasma. This observation suggests that local factors determine the amount of estrogen in tumor tissue and that these determinants are biologically important. We propose to evaluate systematically the determinants of tissue estrogen levels by studying the rate and significance of in situ estrogen formation, of tissue uptake, and of estrogen metabolism in breast cancers from patients and from experimental animals. Our ongoing laboratory program makes available 5-7 human breast tumors/week. We will utilize these to quantitate the levels of aromatase, sulfatase and 17Beta-hydroxysteroid dehydrogenase, the major pathways of estrogen production in tissue. In vitro studies of tissue metabolism will include assay of total estradiol degradation, 2/4 estrogen hydroxylase, sulfotransferase, and glucuronidase. Ongoing clinical trials of endocrine therapy provide an opportunity to evaluate the significance of in vitro data by conducting in vivo studies. We will infuse tracer amounts of estrogen into patients with breast carcinoma. Notably, we will apply a direct in vivo method utilizing the principle of isotope dilution, to quantitate the fraction of estrogen made locally by tumor tissue in women. At the same time, the amount of estrogen concentrated in tumor tissue after uptake from plasma will be determined and correlated with receptor occupancy. Long range correlations of these biochemical data and clinical responses to endocrine therapy should provide new markers for predicting the hormone-dependence of breast cancer. The studies proposed combine a comprehensive biochemical evaluation of estrogen metabolism in tumor tissue in vitro with in vivo steroid infusion studies in patients and in experimental animals and are structured to allow clinical correlations during long-term follow up. Thus, the disciplines of endocrinology, oncology and biochemistry will be combined to study the most frequent cancer found in adult women in the United States.